Visual display systems are commonly used to simulate training environments where training through actual operations would be dangerous, expensive or otherwise impractical. One common application for visual display systems is flight simulation. Conventional flight simulators typically include one or more video display screens onto which video images are projected by one or more projectors, such as cathode ray tubes (CRTs). Flight simulators also usually include a control panel and a joystick for providing input to the visual display system in response to the displayed video images. The control panel and surrounding pilot environment are often realistic simulations of the controls and displays present in the actual aircraft. Thus, the operator can simulate the flight of an aircraft and can respond to the environment as depicted by the visual display. One primary objective of flight simulators is to enhance and optimize the simulated images to present the operator with a high fidelity and realistic training environment.
Pilots often use Night Vision Goggles (NVGs) to enhance vision during night operations. The U.S. military currently uses night vision goggles which intensify light in the near infrared portion of the spectrum. Modern night vision goggles are an accessory to the aviator's helmet and are lightweight, self-contained and battery-operated. By tuning the spectral response of the night vision goggles, compatibility with the greenish-blue aircraft cockpit illumination has been achieved and full utilization made of the longer, reddish wavelengths, which predominate in the light that falls naturally on the earth in the nighttime. Night vision goggles have luminous gain in excess of 100,000 foot lamberts per foot candle and provide resolution and contrast under some conditions approaching that of the unaided eye in daylight. Increased emphasis on night operations coupled with the trend toward reduction in flying time have led to growing concerns about safety and significant need for high fidelity NVG training, particularly in the military sector.
There are two basic types of existing NVG visual display systems for use in training: NVG simulation systems and NVG stimulation systems. Simulation systems provide fully simulated NVG imagery projected onto displays and do not allow the trainee to use actual night vision goggles. Stimulation systems, on the other hand, use a light generated display to artificially stimulate actual night vision goggles to react as they would in true nighttime operations. It is recognized in the field, particularly by military training personnel, that existing NVG display systems of both types are inadequate.
Prior systems that have unsuccessfully attempted to provide full and realistic NVG training include large dome systems, such as the McDonnell Douglas developed Night Attack Weapons Systems Trainer (stimulation type) and the Evans and Sutherland VistaView.TM. (simulation type); collimated flight simulation displays such as mirror beam splitter and off-axis display systems; helmet-mounted direct NVG stimulators such as the Evans and Sutherland NiteView.TM. system; and various helmet-mounted simulated displays.
These previously existing NVG training systems are incapable of providing full and realistic NVG imagery, do not accurately replicate the fit and feel of actual night vision goggles, and/or fail to fully integrate non-NVG enhanced imagery, such as instrument panels, cockpit lighting, and out-of-cockpit imagery visible to the unaided eye by looking around the goggles. Stimulation systems operating with large dome displays do not provide the wide dynamic range necessary to replicate the optical effects encountered when using night vision goggles in actual operations. Collimated displays have low fields of regard, generally considered insufficient for many applications, such as realistic fighter pilot simulation. Helmet-mounted systems are heavy and cumbersome and restrict the operator's head movement, provide no interaction with the crew station, and do not allow full viewing of instrument panels, cockpit lighting, and out-of-cockpit imagery. In addition to the fit and crew station integration problems inherent in all helmet mounted systems, helmet mounted simulated displays cannot provide an actual view of the aircraft heads up display ("HUD") and must incorporate a simulated HUD into the simulated NVG display. The resolution of the miniature CRTs used in simulated NVG displays is significantly less than the resolution of actual night vision goggles. This reduction in resolution results in a lack of realism, particularly for HUD simulation.
Rear projection systems are favored for daytime training because they produce high fidelity imagery within a large field of regard, but, until now, have been incapable of providing realistic NVG stimulation. Rear projection displays have come into favor for use in visual systems for daytime tactical training because of their high contrast and suitability for forming very large fields of regard by juxtaposing multiple screens. These characteristics are also highly desirable for NVG training. However, developers of NVG training systems have heretofore been unable to overcome two basic problems associated with NVG stimulation utilizing rear projection display, namely providing extremely wide dynamic range and adequate depth of focus.
Developers of NVG stimulation systems utilizing rear projection have previously been unable to create a system that provides extremely wide dynamic range (as much as 140 dB) of light intensity typically encountered in the night environment. The range of natural night sky illumination is three orders of magnitude from overcast sky to a clear night sky (full moon). In addition to the natural night sky ambient illumination, pilots are exposed to directly viewed light sources, including artificial light sources such as street lights, airfield lighting, car headlights, weapon effects, explosions, and NVG jammers. On a single night mission an aircrew may encounter extremely low ambient light conditions of the natural night sky and very bright artificial light sources in the same field of view. Extremely wide dynamic range is critical to reproducing this full range of illumination for realistic nighttime training.
In addition, bright artificial light sources create what is commonly referred to as the halo effect when viewed through night vision goggles. That is, when bright point light sources are directly viewed through night vision goggles, the pilot sees a bright central dot surrounded by a larger, circular glow. This "halo" reduces the pilot's ability to distinguish detail in the background areas of the scene. Developers have thus far been unable to produce rear projection NVG system with a dynamic range of at least 140 db. Tests have shown that 140 db is the minimum required to portray the irradiance of the naturally illuminated nighttime scene, and also accurately reproduce the halo effect by stimulating the night vision goggles as if by bright artificial light sources.
A second basic problem that developers of rear projection NVG systems have previously been unable to overcome is providing sufficient depth of focus when attempting to stimulate night vision goggles using close proximity rear projection displays. Because flat panels are used in these displays, the viewing distance is relatively short and tends to vary by a large amount between the center and extremes of each panel. Commonly used types of night vision goggles have objective lenses, which can be focused to the nominal viewing distance. However, the fast objectives required by night vision goggles for maximum light gathering cannot maintain focus for wide variations in object distance. Previous attempts to solve this problem by the obvious method of reducing the diameter of the entrance pupil to the NVG objective lens have been effective in increasing the depth of focus, but at the cost of further limiting the already marginal dynamic range of the display.
For the foregoing reasons, there is a strong need for a system and method for full and realistic stimulation of night vision goggles that provides extremely wide dynamic range and adequate depth of focus. Preferably, the system would be compatible with high fidelity daytime visual displays utilizing rear screen projection and would allow the pilot to use substantially unmodified, conventional night vision goggles.